The present invention refers to a racket for striking a ball, and in particular to a tennis racket of the type having a closed inner stringed frame and an outer support frame which at least partly surrounds the inner frame and is connected to a handle, with both frames being connected at various points.
A two-framed racket of this type is known from German publication DE-OS 27 25 471 which describes a racket with an inner stringed frame detachably secured to a bifurcated shaft/handle portion via vibration-absorbing buffer elements at various locations. The buffer elements are spaced essentially symmetrical to the transverse center plane of the frame, with the number of buffer elements varying and representing the main variable. By modifying the hardness of the flexible buffer elements (silent blocks), the frequency band of filtered and absorbed oscillations of higher frequency is to be regulated. This represents the second variable. A typical characteristic for this design is the dissipation and conversion of high frequency oscillation energy in deformation energy and thus the irreversible conversion in heat energy.
The multipoint connection by means of block-shaped buffer elements with their known linear-elastic spring characteristics allows only a very limited relative movement through vibration of the thus flexibly linked subsystems. These buffer elements absorb vibrations which would otherwise propagate to the shaft/handle portion. The design of the buffer elements is only restricted by the requirement of converting a vibration of the inner frame in a deformation of one part of the buffer element.
The teaching of the racket according to the German publication DE-OS 27 25 471 is based on dynamic absorption of vibrations in the buffer elements, with the frequency band to be absorbed being controlled by the type of buffer elements and only restricted to higher frequencies. This publication is silent as to the design of the racket head in respect to profile, positions of the center of gravity, and absolute and relative positioning of the support between inner and outer frames.
German publication DE-OS 21 16 920 discloses a racket in which an elastic movement of the racket head relative to the handle is attained by maintaining the angular position of the racket head relative to the handle through arrangement of elastic members--primarily pin-supported coil springs outside the racket head. A constant in all embodiments disclosed in this prior art is the fact that the inner and outer frame planes remain parallel during the relative movement. The difference between various embodiments resides in the impact of the elastic members on individual or grouped, partly elastically linked strings.
Coil springs have a linear-elastic spring characteristic. Because of the plurality of springs and the limited space and spring travel, the springs in their entirety define a spring-mass system with higher eigenfrequency and smaller amplitude of oscillation. A particular feature of this racket resides in the force transfer from inner frame to outer frame along the longitudinal frame perimeter. In particular, the springs have to be positioned at the crown-near side and handle-near side of the racket head and should have sufficient stiffness in order to retain the angular position when balls impact in these areas.
Impulses and impact forces are thus transmitted essentially via the entire outer frame area into the handle. In the event this racket uses pins, the latter serve for connecting the elastic members and the filaments of the strings.
A racket of this type has thus a multipoint support of the frame parts, with resilient coil springs/leaf springs allowing a purely translational and limited relative movement. Vibrations are absorbed essentially by internal friction at harmonic expansion i.e. through stiffness damping. Frame and elastic members suffer irreversible thermal losses (heat) during work.
British patent specification No. 431,394 discloses a racket in which the racket head is detachably mounted by elements is proposed in order to establish the desired angular position of the racket head relative to the handle. These resilient members or elastic members include springs or elastic strips or strings and should compensate for the reduced resiliency of the stringing at the racket head ends.
According to the British publication, two pivot pins define a common swivel axis, with a spring element being provided between the frames in the handle-near area in longitudinal direction of the racket, and the inner frame is allowed to carry out a rigid-body rotation about the axis of rotation of the pivot pins regardless of other existing flexibility of the racket parts. However, the handle-near spring element restricts the rigid-body rotation because during relative motion of the frames, the line of action of the compression-tension resiliency is directed toward the racket plane and thus a return moment about the axis of rotation is built up. Especially an assumed perfectly rigid racket head would perform a limited rigid-body rotation relative to the handle.